Bone grafting is a common procedure performed by orthopedic trauma and spinal surgeons to regenerate tissue at a site affected by trauma, disease, or deformity. Graft materials include bone taken from the patient's own skeleton, allograft bone derived from tissue donors, and synthetics that consist of porous ceramics and polymers. Demineralized bone matrix (DBM), is a special type of bone graft that actively stimulates bone formation due to the presence of growth factors within the DBM particles. Derived from allogeneic cortical bone, DBM has been widely used in a variety of orthopedic cases. However, the particulate nature of DBM has made its delivery and containment difficult.
To improve the handling properties of DBM, several companies have developed products that combine DBM particles with a gel-like carrier to try to improve the handling and graft containment of DBM at the surgical site. However, not all carriers perform equally well as a DBM delivery vehicle. For example, the combination of glycerol and DBM, disclosed in U.S. Pat. No. 5,073,373, results in a moldable putty, however, the glycerol component is highly soluble in water and body fluids, and can readily wash away from the graft site. The combination of a thermally reversible copolymer and DBM, disclosed in U.S. Pat. No. 6,309,659, improves upon the solubility of the carrier, however, studies have shown that the final product has variable biological activity [Wang J C, et al. “Prospective comparison of commercially available demineralized bone matrix for spinal fusion”. Trans. North Am. Spine Soc 2000; 15:35-37]. Another DBM product, disclosed in U.S. Pat. No. 6,652,887, is a combination of calcium sulfate, carboxymethylcellulose, water, and DBM. Although this product handles well and is resistant to irrigation, the DBM is enclosed in a carrier that disadvantageously takes 6 to 8 weeks to resorb. With typical graft incorporation occurring within the first 72 hours, the delayed resorption of the carrier can interfere with the healing response. [Lee Y P, Wang J C, Kanim L E, Jo M J, Davis M, Lieberman J R. “The direct comparison of different commercially available demineralized bone matrix substances in an athymic rat model.” Trans North Am Spine Soc 2001; 16:86-87].
The use of bone in gelatin manufacturing has been a widely used process since the early 1900s. By weight, bone is approximately 70% mineral, 20% collagen, 5% growth factors, and 5% water [Bostrom, M P, Boskey A, Kaufman J K, Einhom T A. “Form and Function of Bone.” Orthopaedic Basic Science. Buckwalter J A, Einhorn T A, Simon S R. (eds). Rosemont: American Academy of Orthopaedic Surgeons. pp. 327-328 (2000)]. Through various processing methods, the cross-linked collagen component of bone can be broken down to create gelatin. The gelatin molecule consists of collagen fragments that interact together to form rigid and semi rigid gels at room temperature and below. Standard gelatin manufacturing processes using bone as a raw material, start by treating the bone with an acidic solution to dissolve the bone mineral. The remaining material called ossein is composed of 80% collagen by dry weight. A combination of acids, bases, and high temperature can be used to break apart the collagen molecules in ossein to create gelatin fragments. Done in several extraction steps, a range of purified gelatin solutions can be obtained. Similar processing has been disclosed to create gelatin from human bone to be used as a carrier for DBM. All of these techniques use DBM as a starting material. Although DBM and ossein are chemically similar, DBM processing is slightly different in that care must be taken to maintain biological activity of the growth factors found in bone.
In commercial gelatin production, more aggressive processes are used during demineralization because the goal is to eventually extract only purified gelatin. Gendler et al. followed standard gelatin processing and discloses a method for treating DBM with high temperatures and pressures to thermally degrade the collagen to create gelatin (U.S. Pat. No. 6,576,249). This was accomplished using autoclaves and other pressure vessels. Although the biological activity of the DBM is destroyed, active DBM is added back to the gelatin to create a DBM putty. Although human gelatin is formed using the Gendler process, the single step exposure of DBM to pressure and temperature produces gels of low quality with varying properties. The resulting DBM bone graft putties have poor handling and vary from lot to lot.
Kay et al. used a gentler approach to extracting gelatin by treating DBM with acids, bases, and/or salts to chemically degrade the collagen at room temperature and below (US Published Patent Application 2003/0044445). In this process, the room temperature extraction can maintain some of the biological activity of the growth factor content of DBM. However, the low temperature extraction limits the amount and quality of gelatin produced from the extraction process. In both techniques, DBM is used as a starting material, and the resulting gel is composed of protein fragments from collagen and other proteins found in human DBM. Bone graft materials manufactured from these gels are formed by combining the DBM-derived gelatin with DBM particles.